Fool on street is landing (9)
I believe the answer is:
Here is my best explanation:
'is landing' is the definition.
'fool on street' is the subsidiary indication.
'fool' becomes 'airhead' (**).
'on' says to put letters next to each other.
'street' becomes 'st'.
'airhead' after 'st' is 'stairhead'.
(Other definitions for stairhead that I've seen before include "landing after a flight" and "Landing".)